The present invention relates to a ceramic composition containing a polyacetal binder suitable for injection molding ceramic components. More particularly, the present invention relates to ceramic composites and the preparation thereof by injection molding a ceramic composition containing a polyacetal binder and silicon carbide whiskers which are uniformly dispersed within the composition and the final ceramic composite.
Ceramic materials are of critical importance for a number of high temperature, high performance applications. Recently, there has been substantial interest in the development of ceramic compositions for critical engine parts including reciprocating engines, gas turbine and rocket engines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness. However, the inability to produce complex shapes of high dimensional accuracy and sufficient strength using an economical fabrication technique has prevented ceramic materials from fulfilling their potential in these critical high temperature, high performance applications.
Several processes have been used in an attempt to form ceramic bodies. Among such processes include pressing ceramic powder into a greenbody followed by sintering or by hot pressing and subsequently shaping or machining the sintered body to produce the finished product. Another technique is slip casting in which the ceramic particles are dispersed in water, the slurry placed in a mold and the water removed to form a greenbody. The pressing techniques have been found unsuitable to form ceramic articles of complex shapes and which must meet specific design specifications. The slip casting technique is time consuming and has not yielded greenbodies of sufficient strength.
In view of the problems associated with the prior techniques, injection molding has been increasingly used to form ceramic articles. Injection molding is a process wherein a moldable composition is forced into a mold or die. The injection molding process facilitates a rapid and repeated forming of a plurality of articles having a consistency with close dimensional tolerance. The injection molding process also minimizes the amount of shaping or machining that may be required to produce a finished article.
The injection molding process typically involves forming a ceramic greenbody by injection molding a composition comprising ceramic powder dispersed within a thermoplastic polymer, burning out the polymer from the green body, and sintering the resulting porous greenbody to a dense ceramic part with the same shape. The thermoplastic binder acts as a fluidizing agent to distribute the injection pressure throughout the mold and as the material which holds the ceramic particles in the shape of the mold after the part is ejected. A typical ceramic powder/thermoplastic polymer composite has a very high content of the ceramic particles, typically from about 50 to about 87 volume % and a minimum of the binder material to the hold the particles together in desired shape. A useful binder material for ceramic injection molding is a polyacetal resin as disclosed in U.S. Pat. No. 4,624,812, the entire contents of which are herein incorporated by reference.
A typical injection moldable ceramic composition will also contain a minor binder component which is often a thermoplastic, wax or oil, plasticizers which increase the fluidity of the ceramic-binder mixture, and processing aids such as surfactants which improve the wetting characteristics between the plastic binder and ceramic during mixing to form the composite.
A summary of injection molding applied to the fabrication of molded ceramic bodies is provided in an article entitled "Review: Fabrication of Engineering Ceramics by Injection Molding. I. Materials Selection", M. J. Edirisinghe et al, International Journal of High Technology Ceramics, Vol. II, 1986, pp. 1-31.
For use in high temperature, high performance applications, fracture toughness of the ceramic composite is a critical consideration. Conventional ceramic materials have relatively low fracture toughness with the exception of Al.sub.2 O.sub.3 --ZrO.sub.2 and partially stabilized ZrO.sub.2. Utilization of monolithic ceramic material such as SiC, Si.sub.3 N.sub.4, Al.sub.2 O.sub.3 and mullite (3Al.sub.2 O.sub.3.2SiO.sub.2) for the fabrication of structural components for use in heat engines and other high-temperature conversion systems has required the use of ceramic components with very small flaw size, e.g., less than about 50 m, in order to provide acceptable fracture toughness. However, in structural components especially of complex configuration, the determination of such small flaw sizes has been very difficult to achieve by using nondestructive inspection techniques.
Efforts to overcome the lack of sufficient fracture toughness in ceramic material have included the development of whisker-reinforced composites. The use of the single crystal silicon carbide whiskers in the ceramic composite has been found to improve the fracture toughness of the ceramic composite due to the ability of the whiskers to absorb cracking energy. The whiskers appear to toughen the composites by crack deflection, as when a crack encounters the whisker, crack bridging and by whisker "pull-out". Whisker "pull-out" occurs during cracking of the ceramic matrix at the SiC whisker-matrix interface where shear strength is relatively low as provided by radial tensile stresses across the whisker-matrix bond. As a crack-front propagates into the composite, many of the whiskers which span the crack line and extend into the ceramic matrix on opposite sides of the crack must be either fractured or pulled out of the matrix in order for the crack to grow or propagate through the ceramic. Since the single crystal SiC whiskers possess sufficient tensile strength to resist fracturing, they must be pulled out of the matrix for the crack to propagate. As these whiskers are pulled out of the matrix, they exhibit considerable bridging forces on the face of the crack and effectively reduce the stress intensity at the crack tip so as to absorb the cracking energy. Whisker pull-out, accordingly, effectively reduces the tendency of the composite to crack and also inhibits crack propagation. U.S. Pat. Nos. 4,543,345; 4,569,886 and 4,657,877 disclose silicon carbide whisker-reinforced ceramic composites.
Unfortunately, silicon carbide whisker-reinforced ceramic composites have only shown limited improvements in fracture toughness over the unreinforced ceramic. For example, alumina has a fracture toughness of about 4 MPa.m.sup.1/2 while SiC whisker reinforced alumina has a fracture toughness of about 8-10 mPa.m.sup.1/2. Continuous fiber alumina composites have a fracture toughness as high as 25 MPa.m.sup.1/2. One possible reason for the unsatisfactory improvement is that the fracture strength of the composite is limited by the nonuniform distribution of the whiskers within the composite. Typical methods of mixing and dispersing the SiC whiskers within the ceramic powders involve mixing in a liquid medium such as alcohol or water with the use of a high shear ultrasonic homogenizer. More elaborate sedimentation techniques have also been used in an attempt to uniformly disperse the whiskers within the ceramic powder mix. Unfortunately, due to the size and shape of the whiskers, and particularly to the broad aspect ratio distribution and large aspect ratios (length/width) the whiskers as received and as produced are found as agglomerates and form clumps often called "nests". None of the mixing techniques has found much success in providing a homogeneous dispersion of the whiskers in the ceramic matrix. Agglomeration of the whiskers and the consequent local nonuniform densification of the composite are still observed, which result in large defects in the composite and/or regions of low fracture toughness. Additionally, in view of the severe whisker clumping which takes place and difficulty in forming a uniform mix of whiskers and ceramic powder, reduced levels of the whiskers must be used to form the ceramic composites. Obviously, substantial improvements in fracture toughness of the ceramic cannot be obtained if there is an insufficient amount of whisker loading.
Accordingly, improvements in whisker dispersion would be expected to provide for improvements in the strength and toughness of SiC whisker-reinforced ceramic composites. Such improvements form the basis and primary objective of the present invention.